A fusion bomb is essentially a miniature, brief sun. These bombs are also known as 'H', Hydrogen or thermonuclear devices.

These don't work by splitting atoms. Instead, they do the contrary and actually produce new atoms during the process.

Fusion reactions are the opposite to fission processes. During fission, atoms split apart and get degrade into elements with a lower atomic number. During fusion, lighter atoms are transformed into heavier ones.

What happens inside an 'H' bomb is that Hydrogen atoms are being recombined to form Helium.

Fusion reactions are by far, more powerful than fission processes. They leave less residual radiation and reach much higher temperatures, a set of characteristics which make thermonuclear devices especially apt for strategic purposes, because these devices could easily reach destructive radii of 100 kilometres.

Since temperatures reached during the reaction surpass a million degrees Celsius, destruction in the target area is assured. In other words, if you throw a thermonuclear device over a big city, nothing will be left.

These warheads are not easy to produce and operate. Moreover: fusion processes need extremely high levels of energy just to start, and thus, all hydrogen bombs are in reality comprised of two nuclear devices: the 'H' component itself and a fission starter which provides the initial energy for the thermonuclear reaction to start.

Given their destructive power and the complicated procedures required to operate such warheads, they found their niche among strategic weapons, and it is unlikely that private parties such as terrorist groups will find a use for them.